Hydraulic power unit



June 23, 1953 E. H. BOWERS 2,642,304

HYDRAU I POWER UNIT Filed March 27. 195] Patented June 23, 1953 HYDRAULIC POWER UNIT Eric H. Bowers, Cheltenham, England, assignor to Dowty Equipment Limited, Cheltenham, England Application March 27, 1951, Seiial'No. 217,692 ,7

In Great Britain March 31, 1950 i The present invention consists in a hydraulic power unit, which has been devised more especially for feeding hydraulically actuated services installed in aircraft, and which comprises fixed displacement types of low and high pressure pumps constituting first and second stages, respectively, the low-pressure pump being of greater capacity than the high pressure pump and having an associated relief-valve-controlled by-pass extending between its delivery and suction sides; a fiow communicating passage between the delivery side of the low pressure pump and the suction side of the high-pressure'pump; and a spring loaded shutoff valve which is responsive to a predetermined pressure in the delivery line of the high-pressure pump whereby to move to close or restrict said flow communication and thus to cut off or reduce the supply of liquid from the low-pressure pump to the'highs pressure pump.

If the shut-off valve moves to restrict the communication between the two pumps, saidshut-off valve is preferably arranged so that it moves initially to restrict the communication'and moves subsequently to open a pressure-limiting escape passage extending from the delivery side of the high-pressure pump whilst maintaining the communication between the two pumps restricted.

The relief valve controlling the by-pass across the low-pressure pump will preferably be arranged to function at two pressures, i. e. at one pressure when the high-pressure pump is on load and isreceiving its maximum amount of liquid from the low-pressure pump, and at another, reduced, pressure when the high-pressure pump is oil load and is receiving lessliquid from the low-pressure pump. Thus the spring loading on the relief valve may be in two stages operating in series, one of which is operative when the high-pressure pump is on load, and both of which arev operative when the high-pressure,

pump is off load. The changeover from the 8 Claims. (01. 1os -s)- effectiveness of one to both stages of relief valve spring-loading, and vice versa, may be controlled from the suction side of the high-pressure pump.

By placing the suction side in communicationv with the outer end of a piston member separating the two stages of spring-loading, thepressure in the suction side when the high-pressure pump is on load will augment onest'age of the springing and form in effect a rigid abutment against which the other stage can compress.

When the high-pressure pump is off load and the communication from'the low-pressure pump is restricted, the fall in pressure in the suction side will permit the second stage of therelief substantially constant irrespective of variations in the inlet pressure of the low-pressurepump.

I "The low-pressure pump may be agear type pump, and the high-pressure. pump a radial cylinder type pump, the two pumps may be coupled to be driven froma common driving shaft.

In order that the invention may be more clearly understood and readily carried into effect, a hydraulic power unit in accordance therewith will now be described by way of example with reference to the diagrammatic drawing accompanying this specification, wherein the components f such a'unittogether with the various connecting passages are shown in section.

' The unit shown has the various parts'mounted within a casing I I having a main inlet connection l2, a'delivery line connection 13 and a return connection I 4. A low pressure fixed displacement gear pump l5 draws liquid through the inlet opening l2 and delivers it through an intermediate passage 16- to the suction gallery ll of a radial cylinder pump l8. The pump l8 operates as a, fixed displacement pump and comprises a number of reciprocable plungers l9 operating in stationary radial cylinder bores 20. The plungers I 9 are constrained to reciprocate positively in both directions by an eccentric 21' which isdriven to rotate about the axis of the pump cylinder assembly. The plungers l9 are formed with transfer ports 22 which register with the suction gallery I! when the plungers are innermost, as at the lower centerposition in the drawing. As the-plungers move out'during their delivery strokes they force entrapped liquid past the correspondingoutlet valve 23 into the highpressure delivery gallery 24. The pumps 15 and t8 are coupled together and driven from a common driving shaft, though'they are shown separated in the diagrammaticdrawing for clarity.

The passage 16 is controlled by a shut-off valve 21 which either occupies the position shown at which it permits full-flow along the passage, 01' occupies-an up position at which the flow is restricted bythe metering orifice 28 in a bypass "passage [6b. The shut-off valve 2'! is 3 ..@spring loaded by a coiled compression spring H29 and an extension 44 at its lower end is exposed to pressure in the high-pressure pump devery gallery along a conduit 30. In the po- ;-Siti0n shown the extension 44 seats within a de- -f'pression and fluid at 30 cannot escape, but f when the valve 21 is raised fully the pressure ';,at 30 may pass the extension 44. The shut- .I-off valve 21 has a number of apertures 3| for escape of fluid which passes the raised extension 44, and is arranged in line with the return connection l4 leading to the reservoir of the system. The passage l6 has a branch 34 leading through a restriction 35 to the return outlet l4. The gear pump l5 has an associated by-pass passage 25 controlled by a relief valve indicated 5 generally at 26. The relief valve 26 comprises la valve element 36 and a piston element 31. A compression spring 36 extends between said elements and a further compression spring 33 extends between the outer end of the piston element 31 and the casing H.

The space 40 inside the casing of the pump I6 communicates along a passage 4| with the inlet .;,of the pump 15.

The unit operates in the following manner. The pumps [5 and H! are continuously driven. Assuming that the pump I6 is operating one or I'more services which are connected with the unit at I3, the shut-off valve 21 will occupy the posi- Ation shown in the drawing and the pump [6 will '--be receiving its maximum amount of liquid from *1 the pump [5. surplus liquid from the pump will -return to the inlet l2 by way of the relief valve element 36 and the by-pass 25. When the service or services reach the end of their travel the rise iin pressure in the delivery gallery 24 will be transmitted to the lower end of the extension {lot the shut-oil valve 21 with the result that the "*aj'shut-ofi valve will move up against the action of its spring 23 to close the passage [6 to the full-area section Ilia so that liquid from the pump l5 can reach pump I8 only through the restric- {l tion 28 in the by-pass passage l6b.

When the pressure in the delivery gallery 24 of the high-pressure pump has risen sufliciently, the shut-off valve 21 will be raised still further and permit liquid from the conduit 30 to pass through the apertures 3| in the shut-off valve and out through the return connection I4 to the reservoir. Liquid flowing through the restriction 28 to the pump [8 and circulated back through the apertures 3| will maintain line pressure and will make good any losses there may be in the system.

All the while the full diameter portion l6a is opened and there is full fiow along the passage [6, the pump 13 will be operating at its full capacity and the pressure in the space 42 below the piston element 31 of the relief valve 26, being subjected by way of passages 45 and I6, I6a to the full outlet pressure of the first stage pump, will be sufiicient to overcome the action of the spring 38 extending between the piston element 31 and the valve element 36 and will retain the piston element 31 fully upraised and the spring 38 highly compressed. The valve element 36 will therefore relieve at its "high pressure. As soon as the passage He has been closed by upward movement of the shut-oil valve 21 so that the flow will be limited to that which the restriction 26 can pass pressure in the suction gallery l1 will fall as will the pressure in the communicating space 42 of the relief valve. The piston element 31 will then be free to move towards the space 42 under the expansive force of the spring 38.

The spring 38, thus extended, will enable the relief valve to operate at its low pressure and the first-stage pump IE will be operating in its off-load condition. This will be the condition which will prevail during idling or off-load condition of the high-pressure pump it, as the main delivery line from the connection l3 will be closed by the selector valve associated with the hydraulically actuated services. The constant escape of some of the liquid through the restriction 35 will aid cooling of the unit by increasing the quantity of cool oil constantly entering the unit.

The unit above described provides a ready source of hydraulic power which can be drawn upon whenever needed, and the valve mechanism between the two stages of the unit will ensure that the high pressure pump will operate at capacity only when such operation is required. Moreover, both pumps are able to operate in an oil-load condition with a minimum power input.

I claim:

1. A hydraulic pump unit comprising a fixeddisplacement first stage pump, a fixed-displacement second stage pump of lesser volumetric displacement than said first stage pump, driven in common with the latter, a by-pass passage and a relief valve therein interconnecting the outlet and the inlet of the first stage pump, an intermediate passage of a size to convey the entire intake capacity of the second stage pump, connecting the outlet of the first stage pump and the inlet of the second stage pump, a shut-off valve interposed in said intermediate passage and biased towards open position, pressure-responsive means operatively connected to the shut-off valve to close the same, a high pressure passage leading to said pressure-responsive means from the outlet of the second stage pump for actuation of the pressure-responsive means, and consequent closure of the shut-off valve, upon attainment of a predetermined maximum pressure in such outlet, such as corresponds to continued operation of the pumps under substantially no-load conditions, and a flow-restricting passage connecting difierent parts of the length of said intermediate passage and by-passing said shut-ofi valve to provide a continuous fiow of liquid to the second stage pump, less than its full intake capacity, notwithstanding closure of said shut-oil valve.

2. A hydraulic pump unit as in claim 1, including restricted ports open under control of the shut-off valve, only when that valve is fully closed, for communication between the high pressure passage and a low pressure outlet when the pressure-responsive means and the shut-off valve reach full-closed position, corresponding to continued operation of the pumps under substantially no-load conditions.

3. A hydraulic pump unit as in claim 1, including a restricted passage continuously connecting the outlet of the first stage pump to a low pressure outlet, to provide a continuous flow of liquid into and through the first stage pump notwithstanding closure of the shut-off valve.

4. A hydraulic pump unit comprising a fixeddisplacement first stage pump, a fixed-displacement second stage pump of lesser volumetric displacement than said first stage pump, driven in common with the latter, a by-pass passage and a relief valve therein interconnecting the outlet and the inlet of the first stage pump, an intermediate passage of a size to convey the entire intake capacity of the second stage pump, connecting the outlet of the first stage pump and the inlet of the second stage pump, a shut-off valve interposed in said intermediate passage and biased towards open position, pressure-responsive means operatively connected to the shutoff valve to close the same, and to the outlet of the second stage pump for actuation thereby upon attainment of a predetermined maximum pressure in such outlet, such as accompanies continued operation of the second stage pump under no-load conditions, restricted passages for connecting the outlets of the respective pumps to a low pressure outlet, and a restricted passage connecting the outlet of the first stage pump to the inlet of the second stage pump, for continuous flow of liquid through the two pumps during and notwithstanding closure of the shut-off valve.

5. A hydraulic pump unit as in claim 1, wherein said pressure-responsive means comprises a pressure-actuated plunger bearing against the shut-off valve and seated within a recess in said high pressure passage in rest position under the bias of said shut-off valve, but for movement of the plunger from said recess under the selected maximum pressure to a blow-off position, to impart a corresponding full-opening movement to said shut-off valve, said shut-off valve being proportioned, relative to said recess, to effect closure of the intermediate passage before the plunger attains its blow-off position. I

6. A hydraulic pump unit as in claim 5, including restricted ports in the shut-ofi valve, for communication between the high pressure passage and. a low pressure outlet, through said recess, but closed against leakage so long as the plunger remains in said recess, and when open afi'ording in conjunction with the flow-restricting passage admitting to the second stage pump a continuous-flow path for fluid through said second stage pump during its operation under substantially no-load conditions.

7. A hydraulic pump unit as in claim 1, wherein the relief valve for the first stage pumpincludes a valve body and cooperating seat, a spring bearing on said valve body to retain it normally seated, a pressure-sensitive abutment against which said spring reacts, mounted for shifting to vary the spring force, anda branch passage connected to the intermediate passage downstream from the relief valve and from the flow-restricting passage and from the shut-off valve, and thus subject alternatively to the full outlet pressure of the first stage pump or to the restricted flow and lessened pressure thereof, said branch passage communicating with said abutment to shift the latter in accordance with such difierent pressures.

8. A hydraulic pump unit a in claim 7, including a further passage including a restriction and connecting the outlet of said first stage pump upstream of the shut-off valve with a low pressure outlet.

ERIC H. BOWERS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,770,297 Bussmann July 8, 1930 2,214,390 Whalmark Sept. 10, 1940, 2,295,833 Deschamps Sept. 15, 1942 2,443,527 Wirth June 15, 1948 2,485,753 Lefler Oct. 25, 1949 

